My Canon i560 periodically goes into a loop mode, printing the same
page over and over. (It's done this since I've owned it.) I've been
able to clear this by re-booting and/or re-installing the printer
driver. But this current time, I can't.

This current loop works like this: the printer keeps a former header
memory (header from a website page I printed). It does start print a
new doc, but with that old header. It also prints that page wrongly,
with anomalies, and keeps printing it page indefinitely.

Is this simply the sign of a printer that's defunct? It's 4 or 5 years
old, near as I can recall.

Do you see this behaviour only when the print command is given from a
particular PC or with all the PCs?

I suspect some kind of virus in your system.

You make a very helpful observation--tho the situation is still
inconclusive. It indicates that the trouble is with the PC and not the
printer, since I am able to use the printer with no problems from
another computer--a Mac, in fact.

On the other hand: Is the trouble a virus? The PC is operating fine
otherwise and has McAfee security software in use regularly, with an
automatic virus scan daily, plus enabled firewall (most of the time).
No other problems at all...except that it's very slow to open Outlook
messages (5 to 8 seconds) [But they are immediately accessible in the
preview pane, so maybe it's the anti-virus software that slows the
opening].